U.S. officials are increasingly worried that al-Qaeda elements in Syria are working to recruit American jihadis fighting in the conflict to carry out attacks when they return home.

At least 70 Americans have traveled or attempted to travel to Syria to participate in the civil war, where various Sunni insurgent groups are battling Shia militants and Syrian troops fighting on behalf of embattled President Bashar Assad, reports the New York Times. Analysts also believe that an estimated 1,200 Europeans have also traveled to Syrian battlefronts to wage jihad.

On Thursday, F.B.I. Director James B. Comey told a group of reporters during a press conference that the bureau is prioritizing tracking the movements of Americans that have traveled to Syria once they’ve returned home.

The bureau is reportedly conducting “costly round-the-clock surveillance” on Americans who have fought in Syria and returned to the U.S. fearing that the extensive military training and religious indoctrination they received abroad my be used to threaten or attack their fellow citizens in the future.

“We know Al Qaeda is using Syria to identify individuals they can recruit, provide them additional indoctrination so they’re further radicalized, and leverage them into future soldiers, possibly in the U.S.,” a senior counterterrorism official told the Times.

Correction: An earlier version of this story misattributed an anonymous quotation in The New York Times to FBI Director James Comey.